How to create a bumpy rubber material?
You could try something like this. I used a Voronoi texture clamped with a Color Ramp, similar to what you did to make the main dots. I then used another Voronoi Texture, scaled up slightly, and clamped differently with another Color Ramp to get the smaller dots. I also mixed the input vectors of both the large and the small dots with  a Noise Texture to make them less perfectly round.
Lastly, I mixed the whole thing with another (largely scaled up) Noise Texture to give the fine &quot;grain&quot;. I also gave a very small subsurface value to emulate that semi-translucent rubber look.